Text Box Value and Timer

var txt_1 = txt2.text;
var txt_2 = txt3.text;
txt4.text = (txt_2 - txt_1);

What is the alternative way of doing this??? Seeing the text in the txt boxes are numbers correlative to a timer constantly going. Here is the rest of the code:
var timer2:Timer = new Timer(1);//create the timer
var txt_1 = txt2.text;
var txt_2 = txt3.text;
txt4.text = (txt_2 - txt_1);
//var i:Number = .5;

timer2.addEventListener(TimerEvent.TIMER, doNextTween2);

function doNextTween2(event:TimerEvent):void{
var startTime = getTimer();
txt.text = (startTime);

// //if ( btn.height >= 95)
//{
// timer2.removeEventListener(TimerEvent.TIMER, doNextTween2);
//}

}
btn.addEventListener(MouseEvent.MOUSE_OVER, doNextTween);

function doNextTween(evtObj:MouseEvent):void{
timer2.start();
var cool = getTimer();
txt2.text = cool;

}

btn.addEventListener(MouseEvent.MOUSE_OUT, doNextTween1);

function doNextTween1(evtObj:MouseEvent){
var cool2 = getTimer();
txt3.text = cool2;
timer2.stop();

}

I’m basically trying to record the time the mouse is over a movie clip, and I need it to be as accurate as possible, so to the nearest millisecond.